Dc to dc converters are used to convert a first input dc voltage to a second output dc voltage. This is frequently performed incident to generating a regulated dc voltage as the output of a power supply. Such dc to dc conversion typically involves a first conversion from dc to ac, followed by a second conversion from ac to dc. The initial conversion of dc to ac involves repeated switching of the dc input voltage at a high frequency to generate the ac voltage. Once the ac voltage has been generated, the ac voltage is transformed or transferred by a transformer, which can also provide isolation, and then is rectified and filtered to regain a dc output.
When such switching supplies were developed, it was determined that a size savings could be realized by increasing the switching rate. However, because the switching transistors were commonly forced to switch high current at high voltages, as the switching rate increased to above 500 kHz., significant switching losses occurred.
To avoid such losses, it was determined that by switching sinusoidal waveforms one could switch the transistors when either the current or the voltage was zero. This switching technique was implemented by adding an LC network to work in conjunction with the transistor switches to resonantly store and release energy, while the switching frequency was varied to control the output power. This form of controlling the switching elements is termed resonant control and with these techniques, switching frequencies could be extended to beyond one megahertz.
Such a switching arrangement however resulted in increased ohmic losses. This occurred as a result of the sinusoidal waveforms having less energy for a given amplitude than do square waveforms of the same amplitude. Because of this energy decrease it was necessary to increase the peak amplitude of the sinusoidal signal in order to convert the same amount of energy. This increase in peak current meant that the ohmic losses, which increase with the square of the current, could potentially exceed the switching losses for which the resonant switching method was developed to overcome. An additional troublesome feature of resonant switching with sinusoidal waveforms was that with a variable switching frequency, issues relating to noise and power filtering were more difficult to handle.